A metal line may perform the role of an interconnection circuit for transistors, power supply, and signal transfers in an integrated circuit (IC). Recently, aspect ratios of metal lines have been increasing due to new design rules for smaller, more highly integrated semiconductor devices. The width of the metal line has become smaller while the depth has increased (or at least, decreased less). To develop smaller semiconductor devices, therefore, one requirement is to minimize defects caused during the formation of each metal line layer.